FSSs with cross-dipole patch elements (also known as "dichroic surfaces") have been used in the subreflector of high-gain Cassegrain antennas for reflecting a signal in the X band and passing a signal in the S band. Cross-dipole patch elements have also been used for the subreflector design of a tracking and data relay satellite system (TDRSS) to diplex the S and Ku band signals. [V. D. Agrawal and W. A. Imbriale, "Design of a Dichroic Cassegrain Subreflector,"IEEE Trans., Vol. AP-27, pp. 466-473, July 1979.]
The characteristics of an FSS utilizing cross-dipole elements changes drastically as the incident angle is steered from normal to 45.degree.. Thus, a large band separation is required to minimize the RF losses for diplex operations. This is evidenced by the reflection and transmission band ratio (f.sub.r :f.sub.t) being 7:1 for a single screen FSS with cross-dipole elements. [Agrawal, et al., supra] or 4:1 for double screen FSS [G. H. Schennum, "Frequency Selective Surfaces for Multiple Frequency Antennas," Microwave Journal, pp. 55-57, May 1973] Better elements, such as the square or loop elements, are definitely required for space missions that require band separation for as many as four adjacent bands.